1. Technical Field
The present invention relates to photolithographic lens systems and, more specifically, to a method and apparatus for determining reflection lens transmission distribution while separating contributions of the illumination source aperture uniformity from the reflection lens pupil transmission distribution in a lens system having reflective elements, such as for use with extreme ultraviolet (EUV) lithography.
2. Background
Optical photolithography has been widely used in the semiconductor industry in connection with the formation of a wide range of structures in integrated circuit (IC) chips. As device critical dimensions continue to be reduced, lithography has been forced to move from the visible into the UV, deep UV and extreme UV wavelengths. In doing so, the radiation manipulation means has shifted from transmission-based systems to reflection-based systems. Complex systems of reflective optics have become increasingly common for the purpose of improving resolution or depth of focus. The ability to measure and verify the correct distribution of illumination in the pupil plane is as important in reflection-based systems as it is in transmission-based systems.
Uniformity of the illumination at the wafer surface is needed so that the same exposure of photoresist or other radiation-sensitive films is consistently achieved across the entire exposure field. The degree of partial incoherency of the illumination, or more generally the distribution of pupil illumination, must also be constant across the entire exposure field. As tolerances of the printed lithographic patterns become increasingly tight, the requirement that the pupil illumination distribution not vary across the exposure field becomes increasingly important.
Various illumination systems for lithographic lenses have been developed, including those that create complex patterns of pupil illumination to enhance lithographic resolution and/or depth of focus. Illumination patterns, such as dipole, quadrupole, and annular shapes, have been developed to improve the resolution and depth of focus of the image formation. Some of these illumination patterns are particularly suited to enhancing the lithographic performance of specific mask patterns that are exposed on the stepper. When conventional partially coherent illumination is used, the center of the pupil is illuminated uniformly out to a prescribed fraction of the pupil size. In the case of both conventional partially coherent illumination and the more complex off-axis illumination patterns, the consistency of the illumination pattern at every position in the exposure field is critical.
There is a pervasive trend in the art of IC fabrication to increase the density with which various structures are arranged. As a result, there is a corresponding need to increase the resolution capability of lithography systems. One promising alternative to conventional optical lithography is a next-generation lithography technique known as extreme ultraviolet (EUV) lithography where wavelengths in the range of about 11 nm to about 14 nm are used to expose the photoresist layer. For example, using a numerical aperture of about 0.25, a wavelength of about 13.4 nm and a k1 value of about 0.6, it has been proposed that a resolution of about 32 nm can be achieved.
The quality and uniformity of the illumination at the wafer plane can be analyzed and characterized by a variety of techniques, including wafer-plane power meters, analysis of photoresist or other light-sensitive films, etc. In the past, the pupil illumination has been measured by using either a single, relatively large (one to a few millimeters) aperture in the plane of the photomask, or a plurality of pinholes in an array. In both cases, the aperture functions as a pinhole camera and projects a geometrical image of the pupil illumination pattern. However, this method provides only an overall result, and does not separate sources of any observed non-uniformity.
As dimensions of IC components are continually reduced, and as the wavelength of radiation used in photolithography is reduced, effects of non-uniformity in illumination at the wafer plane become increasingly important. In order to remedy such non-uniformity, the source thereof needs to be identified. Therefore, a need exists for a system that can efficiently obtain quantitative measurements of the illumination pattern at the wafer plane of the photolithographic lens system, while separating contributions to non-uniformity originating from various portions of the photolithographic apparatus.